Adhesives having a polyethylene polymer (including polyethylene copolymer) as a main component have been known as a type of the so-called hot-melt adhesives that permit heat bonding, and have been used mainly in the manufacture of electronic apparatuses. These polyethylene adhesives are chemically stable. Their stability has been proved in tests under severe conditions such as Pressure Cooker Test. As an example of such an adhesive, a hot-melt adhesive comprising an ethylene-glycidyl methacrylate copolymer as a polyethylene polymer, and rosin, is described Japanese Unexamined Patent Publication (Kokai) No. 9-25371 (Claims). In this adhesive, the rosin is added as a tackifier which, together with the polar action of glycidyl group in ethylene-glycidyl methacrylate copolymer, enhance adhesion to metallic surfaces. In this adhesive, the heat curing reaction of the rosin and the ethylene-glycidyl methacrylate copolymer is not positively used.
This adhesive can be used as to secure the lead pin of a lead frame of an integrated circuit. After heat bonding the adhesive, it is immersed in a solder bath, and thereafter is left in a high temperature environment (at 230° to 260° C.) for a long period and then used. Therefore, high heat resistance is required for this adhesive. However, above-mentioned patent publication contains no specific teaching regarding improvement of heat resistance.
To increase heat resistance, it maybe advantageous to increase reactivity of the hot-melt adhesive, and the so-called reactive (curing) hot-melt adhesive in which cross linking reaction after adhesion (post-cure) is made possible has been known. For example, Japanese Unexamined Patent Publication (Kokai) No. 5-295126, a silanol condensation type hot-melt adhesive is disclosed where an olefin having silyl group is used for post-cure cross-linking of polyolefin. A moisture curable type hot-melt adhesive is also known which has ethylene copolymer blended with urethane polymer (see Japanese Unexamined Patent Publication (Kokai) No. 4-8786). However, because moisture is needed for cross-linking this material, it is not suited to application to the manufacture of IC packages.
An adhesive polyolefin is known where a polar component such as vinyl acetate, ethylene acrylate, maleic acid, styrene, is introduced by copolymerization into polyolefin to increase adhesive strength (see Japanese Unexamined Patent Publication (Kokai) No. 5-17735, Japanese Unexamined Patent Publication (Kokai) No. 4-227982, Japanese Unexamined Patent Publication (Kokai) No. 2-261876, Japanese Unexamined Patent Publication (Kokai) No. 2-255884, Japanese Unexamined Patent Publication (Kokai) No. 2-180979)7. However, such a polyolefin has very low heat resistance, and amount of polar groups (reactive functional groups) in the polyolefin molecule needs to be increased to obtain sufficient adhesive strength.
As a heat resistant adhesive containing polyethylene, polyethylene blended with epoxy resin and a latent curing agent is known (see Japanese Unexamined Patent Publication (Kokai) No. 63-301283).
Further, an adhesive composition capable of being used in hot-melt condition comprising (a) ethylene-glycidyl (meth)acrylate copolymer as a first polyethylene polymer, (b) ethylene-alkyl (meth)acrylate copolymer as a second polyethylene polymer, and (c) rosin having carboxyl group in the molecule, wherein cross-linking structure is formed between ethylene units in above-mentioned copolymers is also known (see Japanese Unexamined Patent Publication (Kokai) No. 10-316955). The cross-linking structure can be formed by irradiating electron beam to the mixture containing above-mentioned components (a) to (c). With this composition, problems associated with conventional reactive (curing) hot-melt adhesive such as slow cross-linking reaction, gradual deterioration of adhesive strength due to generation of reaction by-product, gelation during heating, etc., can be overcome. It is also advantageous in that it can be formed into film-like adhesive without using solvent.